Why is Charla so much stronger than Kathryn? Is there an energy boost right after coming back? Or do wraiths gradually lose power over time? One could guess she’s getting a boost from her anger but we’ve seen nothing similar from Kathryn.

Well, Kathryn was also a child when she died, and Charla has a husband and, more importantly, her own daughter to save – I’d say she’s not holding anything back.

Also, considering how newly-woken zombies behave according to Sue, maybe there’s a similar thing happening with wraiths; you’re not thinking clearly and instead just act on instinct. It’s when you need to think about surviving after the next sunrise, that you start worrying about not causing untold damage and/or conserving energy.